1. Field of the Invention
The invention pertains to methods for fabricating devices which include thin film substrates of, for example, polyimide, having metal-containing through holes, as well as the resulting devices.
2. Description of the Related Art
Among the substrates currently being used as semiconductor chip carriers, i.e., carriers of semiconductor integrated circuit devices, are thin, electrically insulating, flexible films of organic polymeric material, such as polyimide. Currently, such a thin, flexible film serves to support a relatively fragile lead frame, which includes free-standing inner and outer leads. This lead frame is usually fabricated by depositing, e.g., sputter depositing, a relatively thin layer of chromium, which serves as an adhesive layer, and a relatively thick layer of copper, onto an upper surface of the film. Conventional photolithographic techniques are used to pattern the deposited metal into the desired lead frame configuration, and portions of the polymeric film are etched away to produce the free-standing inner and outer leads. A semiconductor chip is mounted on, and attached to, the polymeric film carrier by bonding, e.g., thermo-compression bonding, the free-standing inner leads to contact pads on the chip. In turn, the resulting chip carrier is mounted on, and attached to, a printed circuit card or printed circuit board by soldering the free-standing outer leads to the card or board.
The above-described thin film chip carriers are usually processed while in a roll format. Because the thermo-compression bonding referred to above is automated, this processing has come to be known as tape automated bonding (TAB).
Recently, a new thin film chip carrier configuration has been proposed in which the chip carrier is provided with a ground plane and/or a power conductor on the back side (the non-chip-bearing side) of the polymeric film, through which a ground potential and/or electrical power is to be supplied to one or more chip components. In addition, in this new configuration, a number of metallized through holes, extending through the thickness of the polymeric film, are to be included, some of which are to serve as electrical connections between certain chip components (to be supplied with a ground potential or electrical power) and the ground plane or power conductor. Others of these metallized through holes are to serve as electrical connections between other chip components and circuitry on a printed circuit card or printed circuit board.
In the new thin film chip carrier configuration, the usual free-standing inner and outer leads are to be replaced with metallized circuit lines on the chip-bearing surface of the polymeric film, which circuit lines adhere to the polymeric film and intersect metallized lands encircling the metallized through holes, or directly intersect the metallized through holes. It is envisioned that one or more chips will be mounted face-down on the chip bearing surface of the polymeric film, using solder balls to provide mechanical and electrical connections between contact pads on the chip or chips and the circuit lines on the polymeric film. Moreover, the resulting chip carrier is to be mounted on a printed circuit card or printed circuit board using solder balls to mechanically and electrically connect certain of the metallized through holes to circuitry on the card or board.
Because it is expected that the new proposed thin film chip carriers will also be processed in a roll format, and because it is envisioned that there will be area arrays of through holes, the corresponding processing has come to be known as area array tape automated bonding (ATAB).
The new proposed thin film chip carrier, described above, presupposes that the through holes in the polymeric films can be metallized using conventional techniques applicable to, for example, epoxy/fiberglass printed circuit boards. In the latter case, metallization of through holes is achieved by first forming a relatively thin seed layer of an essentially pure palladium-tin colloid on the side walls of the through holes, using conventional wet seeding techniques, i.e., by placing the printed circuit board into an appropriate chemical bath. Subsequently, copper is deposited onto the resulting seed layer using conventional electroplating techniques.
Attempts have been made to form thin seed layers of essentially pure palladium-tin colloids on the side walls of through holes in thin polyimide films. However, these attempts have failed because the resulting seed layers exhibited unacceptably low adhesion to the side walls, as did any copper electroplated onto the seed layers.
Thus, those engaged in the development and manufacture of thin film devices such as thin film polymeric chip carriers have sought, thus far without success, techniques for forming seed layers on the side walls of through holes in polymeric films having acceptably high adhesion.